Because of their excellent characteristics, epoxy resins have been widely used in various fields, such as electrical and electronic devices, paints, in the fields of architecture and civil engineering, and adhesives. Especially in the field of electrical and electronic devices, epoxy resins have been widely used as sealing material for electronic components because of their excellent characteristics in electrical insulation and thermal resistance. In recent years, due to tendency for more and more highly integrated semiconductor device, for further reduction of size and thickness of packages, and for higher packaging efficiency, improvement of reliability is strongly required for epoxy resins used in these applications.
Two main approaches have been adopted for improving reliability of epoxy resins. In the first approach, decrease of the content of halogen-containing group which is present in trace amount in epoxy resins has been attempted. It has been known that hydrolysis of a halogen-containing group in epoxy resins results in degradation of electrical insulation and corrosion of lead wires, and thus adversely affects reliability of electronic devices.
In the second approach, reduction of stress and reduction of moisture absorption of hardened epoxy resins have been attempted. Conventionally, an epoxy resin composition that is composed of a cresol novolac type epoxy resin as a epoxy resin, a phenolic resin as a curing agent, and silica or the like as a filler, has been widely used for sealing semiconductor devices. However, although the cresol novolac epoxy resin has excellent thermal resistance, it has a disadvantage that hardened product lacks flexibility so that cracks tend to be produced in the moldings.
In order to obtain an epoxy resin composition with low hygroscopicity, it is desirable to use the so-called filler high-filled type epoxy resin composition that contains a large amount of filler with low hygroscopicity such as silica. However, use of a cresol-novolac resin leads to increase in the viscosity of the epoxy resin composition at the time of molding, and may result in problems such as molding failure or damage to the sealed micro-electronic parts.
Various processes have been proposed to resolve these problems. In the above-mentioned first approach to improving reliability of an epoxy resin composition, that is, in order to reduce a halogen containing group that is present in trace amount in an epoxy resin, a process is disclosed in Japanese Patent Publication No. 61-136513, in which an alkali metal hydroxide is added to react with an epoxy resin in the presence of isobutanol or a solvent consisting of a secondary alcohol. A process is also disclosed in Japanese Patent Publication No. 62-64817, in which an epoxy resin is reacted in the presence of alcohols, alkali metal hydroxide, and a phase transfer catalyst at temperature of 20˜50° C.
Since, in the process as disclosed in the above-mentioned Japanese Patent Publication No. 61-136513, isobutanol or a secondary alcohol is used, reaction of these alcohols with the epoxy group is induced during the reaction and the epoxy equivalent is thereby undesirably increased. Also in the process as disclosed in the above-mentioned Japanese Patent Publication No. 62-64817, although the reaction takes place at relatively low temperature, a phase transfer catalyst is used and reaction of alcohols and the epoxy group is likely to be induced. Use of expensive catalyst is a disadvantage as industrial application.
A process is disclosed in Japanese Patent Publication No. 62-256821, in which halogen content in epoxy resin is decreased by processing a raw epoxy resin which is prepared by reaction of bisphenols obtained by substituting ortho-position of phenolic hydroxide, and epihalohydrin, in an alkaline solution with water content at a predetermined level or lower. Also in Japanese Patent Publication No. 63-268723, a process is disclosed for decreasing hydrolyzable halogen content of epoxy resin by adding aqueous solution of alkali metal hydroxide and hydrophobic solvent to a raw epoxy resin prepared from poly-phenol and epihalohydrin, and by extracting water from the system by azeotrope of the hydrophobic solvent and water to thereby give rise to ring reclosure reaction.
In the process disclosed in the above-mentioned Japanese Patent Publication No. 62-256821, as described in the specification of the Patent Publication, total chlorine content of 600 ppm or lower is difficult to be achieved except for the epoxy resin having the specific structure which is obtained by reaction of bisphenol having substituted ortho-position of phenolic hydroxide, and epihalohydrin. Even when total chlorine content of 600 ppm or lower is achieved, undesirable reactions such as ring opening of the epoxy group may occur, as described in the Patent Publication, so that this method cannot be used for general industrial applications.
In the process disclosed in the above-mentioned Japanese Patent Publication No. 63-268723, due to high water content of the system in the initial stage of the reaction, the epoxy group may react with water in the system using the alkali metal hydroxide as a catalyst, resulting in ring opening of the epoxy group. As a result, α-glycol may be produced, leading to lowering of the epoxy group content. Produced α-glycol has a hydoxide group of primary alcohol with relatively high reactivity. Thus, when an epoxy resin with sufficiently low chlorine content is to be obtained, the hydroxide group of the produced α-glycol may react with the epoxy group, which may results in undesirable polymerization or gel formation.
In the second approach to improving reliability of an epoxy resin composition, that is, in the attempt to decrease stress and moisture absorption of the hardened epoxy resin, a low molecular weight bi-functional epoxy resin composition having two epoxy groups per molecule has been conventionally used effectively. Since cross-link density is low for bi-functional epoxy resin, the hardened product is of low stress, and by using a low molecular weight epoxy resin, it is possible to overcome the problem of molding failure and damage to the sealed minute electronic components even when a large amount of filler is filled in the epoxy resin composition. However, since an epoxy resin composition using such a bi-functional epoxy resin has lower cross-link density as compared to an epoxy resin composition using a multi-functional epoxy resin, it has a disadvantage that the thermal resistance is also degraded.
In order to resolve this problem, a process for production has been known in which alcoholic hydroxide group that is present in the bi-functional epoxy resin is reacted with epichlorohydrin to obtain glycidyl ether, and thus to provide branching structure of glycidyl ether in the molecule. Also a process is disclosed in Japanese Patent Publication No. 04-353517, in which alcoholic hydroxide group in an epoxy resin having three or more phenolic glycidyl ethers in a molecule, is reacted with epichlorohydrin to obtain glycidyl ether and thereby to improve thermal resistance and water resistance. However, although these processes are effective in improving thermal resistance and water resistance, multi-functionalization by introducing an epoxy resin of short chain length such as epichlorohydrin may result in increase of the elastic modulus, and therefore is undesirable from the viewpoint of decreasing stress.
Especially in recent years, due to remarkable progress toward more and more highly integrated semiconductor device, toward reduced size and thickness of packages, and higher packaging efficiency, improvement of reliability is strongly required for epoxy resins used in these applications, and therefore, there is still a strong need for decrease of the content of halogen containing group that is present in trace amount in epoxy resins as well as for reduction of stress and moisture absorption of hardened epoxy resins.